1. Field of the Invention
This invention relates to a method of preparing reforming catalysts of enhanced dispersion consisting of one or more Group VIII noble metals supported on zeolites, preferably a catalyst consisting of platinum on potassium-containing zeolite L. The catalyst herein exhibits improved activity and activity maintenance for light naphtha aromatization over catalysts of lesser dispersion.
2. Discussion of the Relevant References
Several materials have been employed as hydrocarbon conversion catalysts in such processes as reforming, catalytic dewaxing, alkylation, oxidation and hydrocracking. Examples of catalysts useful for this purpose include those materials comprising catalytically active metal such as a Group VIII noble metal and optionally rhenium supported on or impregnated into a carrier.
Among the hydrocarbon conversion processes, catalytic reforming in the presence of hydrogen is one of the most important. Catalytic reforming is a refinery process designed to increase the octane number of naphtha. Typically in this process, the naphtha is passed over a suitable catalyst under reforming conditions, for example elevated temperatures and pressures well known in the industry in the presence of hydrogen gas and a H.sub.2 /hydrocarbon mole ratio of about 2 to 20. This process involves several different types of reactions, including isomerization, dehydrocyclization of paraffins to produce naphthenes and aromatics, dehydrogenation of cyclohexanes and other naphthenes and alkanes, isomerization/dehydrogenation of cyclopentanes, isomerization of normal paraffins to isoparaffins, and hydrocracking. Paraffin isomerization occurs relatively easily, but contributes only a limited improvement in octane number The reforming reactions most important for the production of high octane components are those which produce aromatics.
The ideal reaction scheme minimizes the hydrocracking of long chain paraffins to gaseous hydrocarbons such as methane and ethane to improve the yield and selectivity to more valuable products of the other reforming reactions, particularly dehydrocyclization. Examples of known catalysts useful for reforming include platinum and optionally rhenium or iridium on an alumina support, platinum on type X and Y zeolites, provided the reactants and products are sufficiently small to flow through the pores of the zeolites, and platinum on cation exchanged type L zeolites.
While zeolite L catalysts, usually in their hydrogen form, have been employed as catalytic dewaxing catalysts and in other applications, they are particularly useful in reforming because they decrease the amount of hydrocracking which occurs during reforming. For example, U.S. Pat. No. 4,104,320 discloses that the use of zeolite L as a support increases the selectivity of the reaction for producing aromatic products. This improvement, however, has been made at the expense of catalyst life. U.K. Appln. 82-14147 filed May 14, 1982 to Wortel entitled "Improved Zeolite L" teaches that a highly crystalline zeolite L material having a cylindrical morphology leads to an improved catalyst life for dehydrocyclization reactions over a conventionally prepared zeolite L disclosed in U.S. Pat. No. 3,216 789. Finally, Belg. Pat. Nos. 895,778 and 895,779 disclose use of a barium-exchanged zeolite L catalyst for high yields in reforming, dehydrocyclization, dealkylation and dehydroisomerization.
It is well known that coke deposits may be removed from deactivated catalysts by heating them in the presence of dilute oxygen at a flame-front temperature of 430.degree. to 540.degree. C. This combustion may be preceded by a flushing with hydrogen or nitrogen gas. High temperature decoking leads, however, to loss of surface area of the supported metal particles and to removal of platinum from the zeolite channels, thus resulting in loss of catalyst activity. Thus, after combustion, the catalyst is often subjected to oxychlorination by contact with air and chlorine or a chlorinated compound such as CCl.sub.4 at elevated temperatures. U.K. Patent application G.B. 2,106,413A, priority document, filed in France, on Sept. 9, 1981 to Bernard et al. further teaches that catalyst regeneration is improved by subjecting the catalyst after oxychlorination to a treatment with water and cooling air before the catalyst is reduced, In addition, French. Appl. No. 8000114 to Bernard discloses a hydrogen regeneration technique. These techniques, however, have not been applied to freshly prepared catalysts which are defined for purposes herein as catalysts which have not been contacted with a hydrocarbon feed.